A conventional gas turbine engine includes in serial flow communication a compressor, a combustor, and a turbine. The compressor provides compressed airflow to the combustor wherein it is mixed with fuel and ignited for generating combustion gases which then flow to the turbine which extracts energy therefrom for powering the compressor.
The turbine includes one or more stages with each stage having an annular turbine nozzle for channeling the combustion gases to a plurality of rotor blades. The turbine nozzle includes a plurality of circumferentially spaced stator vanes fixedly joined at their roots and tips to annular, radially inner and outer bands.
Each of the nozzle vanes has an airfoil cross section with a leading edge, a trailing edge, and pressure and suction sides extending therebetween. In one type of turbine nozzle, the trailing edge of one vane is spaced from the suction side of an adjacent vane between its leading and trailing edges to define a throat having a minimum flow area for the combustion gases channeled between adjacent vanes. Adjacent ones of the vanes define individual throat areas and collectively they define a total throat area. These areas are specified by each particular engine design and are critical factors affecting performance and stall margin of the gas turbine engine.
Furthermore, the total throat area is preferably obtained by providing substantially uniform individual throat areas between the adjacent vanes. Variations in throat area between adjacent vanes can provide undesirable aero-mechanical excitation pressure forces which may lead to undesirable vibration of the rotor blades disposed downstream from the nozzle.
There exist numerous methods for manufacturing gas turbine engine turbine nozzles which provide varying degrees of accuracy of the individual and total throat areas. For example, one method utilizes individual vanes having integral inner and outer band segments which are joined together for forming arcuate nozzle segments. A second method utilizes arcuate inner and outer band segments each having a plurality of circumferentially spaced apertures for receiving the roots and tips of the nozzle vanes which are then joined thereto by tack welding and brazing. A third method utilizes individual vanes which are fixed relative to each other and then inner and outer band segments are cast over the roots and tips thereof for forming arcuate nozzle segments. The separate arcuate nozzle segments in these three exemplary methods include two or more vanes, with the segments being conventionally joined together for forming a complete 360.degree. annular turbine nozzle.
In all of these methods of manufacturing the turbine nozzle, each of the individual vanes and inner and outer band segments is separately manufactured and, therefore, subject to inherent manufacturing tolerances. The tolerances are additive and, therefore, stack-up during assembly of the turbine nozzle which adversely affects the ability to achieve relatively small variation in the individual throat areas and in the desired total throat area.
In one typical gas turbine engine design, it is desirable to maintain the individual and total throat areas to within about .+-.1/4%. However, using conventional manufacturing methods, it is not believed that this small tolerance is achievable. In order to appreciate the smallness of this tolerance and the difficulty in obtaining it, examination of a particular method of manufacturing a turbine nozzle will be helpful.
More specifically, one conventional method of manufacturing a turbine nozzle includes a conventionally known leading edge nest for fixturing individual vanes in space during manufacture for obtaining the required throat area. It is known that to fully locate in space a three-dimensional object, such as a nozzle vane, requires six point supports for preventing translation along the three axes of a three axis orthogonal coordinate system (e.g. X, Y, and Z axes) and for preventing rotation about each of the three axes. Accordingly, conventional fixturing devices are used for predeterminedly locating individual nozzle vanes in space relative to a reference datum so that when adjacent vanes are assembled together they are predeterminedly located relative to each other for providing among other things the required throat area therebetween.
In the leading edge nest, a nozzle vane is fixtured, or supported at six points relative to the datum during the manufacturing process. The leading edge nest includes a first pair of radially spaced leading edge supports for opposing yaw of the vane relative to, for example, the chord of the vane. A second pair of radially spaced midchord supports contact the suction side of the vane between the leading and trailing edges thereof for opposing roll of the vane relative to the chord. A radial support radially locates the vane. And, an aft support contacts the suction side of the vane adjacent to the trailing edge for opposing pitch of the vane relative to the radial axis thereof. The six supports also oppose translation of the vane in all three axes. The vane is typically held against or restrained against the six supports by conventional means including spring clamps, such as those used to position hardware during welding, and set screws as appropriate to react machining forces. The leading edge nest provided by the fixturing device, therefore, predeterminedly positions the vane in space relative to the datum and, relative to adjacent ones of the vanes.
Once the vane is fixtured, then the integral bands of the first method may be conventionally machined relative thereto, or the bands including the apertures of the second method may be spot welded and then brazed thereto, or in the third method, the bands may be cast to the vanes.
After a turbine nozzle has been conventionally manufactured, the individual throat areas between adjacent ones of the vanes are measured for determining the uniformity thereof. If the individual throat areas do not meet applicable specifications, they may be conventionally benched, wherein the trailing edges thereof are permanently abrasively ground away in order to adjust the individual throat areas. In this way, excessive stack-up tolerances which result in unacceptable variation in throat areas between adjacent vanes may be accommodated after the initial manufacturing of the turbine nozzle. However, benching is only effective for correcting a certain small amount of deviation in throat area, and is generally ineffective for correcting the total throat area of all the nozzle vanes. Furthermore, in nozzle vanes conventionally coated for improved nozzle life, benching is not possible since the coatings are typically thin and their effectiveness would be unacceptably damaged.
The leading edge nest ensures that the leading edges of adjacent vanes are aligned in a common plane and that the vanes extend in an aft direction therefrom for providing the predetermined converging nozzle between adjacent ones of the vanes ending in the desired throat, and throat area thereof. However, the accuracy of the throat area is a function of the accuracy of the tolerances of the vanes and the assembly thereof. For example, the thickness of an individual vane has a first tolerance, and, if the vanes are conventionally coated on both sides, each of the coatings has a second tolerance. Accordingly, the accuracy of the throat area is directly related to the stack-up of these two tolerances since the leading edge nest uses the aft support on the suction side of the blade and the throat is defined on the pressure side of the blade between the trailing edge and an adjacent vane. In this example, the initial vane casting may have a first tolerance of 5 mils (0.13 mm) on each side thereof, and a conventional coating on each side of the blade may have a second tolerance of 5 mils (0.13 mm) resulting in a total tolerance stack-up of about 20 mils (0.51 mm) for both sides. An exemplary throat area required between adjacent vanes may be defined in part by the distance between the blades of about 0.555 inches (14.1 mm). A 1/4 % tolerance on the throat area would then be about 2.75 mils (0.07 mm). Dividing the 20 mils (0.51 mm) tolerance stack-up by the desired tolerance of 2.75 mils (0.07 mm) results in about a 700% potential error. If the nozzle is fabricated by the second or third method described above, additional stack-up and error will be added to the above error.